Introduce uid.dat (offline PlayerUIDs), fix multiplayer save data persistence (#536)

* fix: fix multiplayer player data mix between different players bug

Fixes a Win64 multiplayer issue where player data (`players/*.dat`) could be mismatched because identity was effectively tied to connection-order `smallId` XUIDs.
Introduces a deterministic username-derived persistent XUID and integrates it into the existing XUID-based save pipeline.

- Added `Windows64_NameXuid` for deterministic `name -> persistent xuid` resolution
- On Win64 login (`PlayerList`), set `ServerPlayer::xuid` from username-based resolver
- Aligned local player `xuid` assignment (`Minecraft`) for create/init/respawn paths to use the same resolver
- Added Win64 local-self guard in `ClientConnection::handleAddPlayer` using name match to avoid duplicate local remote-player creation
- Kept `IQNet::GetPlayerByXuid` compatibility fallback behavior, while extending lookup to also resolve username-based XUIDs
- Moved implementation to `Minecraft.Client/Windows64/Windows64_NameXuid.h`; kept legacy `Win64NameXuid.h` as compatibility include

Rename migration is intentionally out of scope (same-name identity only).

* fix: preserve legacy host xuid (base xuid + 0) for existing world compatibility

- Add legacy embedded host XUID helper (base + 0).
- When Minecraft.Client is hosting, force only the first host player to use legacy host XUID.
- Keep name-based XUID for non-host players.
- Prevent old singleplayer/hosted worlds from losing/mismatching host player data.

* update: migrate Win64 player uid to `uid.dat`-backed XUID and add XUID based duplicate login guards

- Replace Win64 username-derived XUID resolution with persistent `uid.dat`-backed identity (`Windows64_Xuid` / `Win64Xuid`).
- Persist a per-client XUID next to the executable, with first-run generation, read/write, and process-local caching.
- Keep legacy host compatibility by pinning host self to legacy embedded `base + 0` XUID for existing world/playerdata continuity.
- Propagate packet-authoritative XUIDs into QNet player slots via `m_resolvedXuid`, and use it for `GetXuid`/`GetPlayerByXuid` with legacy fallback.
- Update Win64 profile/network paths to use persistent XUID for non-host clients and clear resolved identity on disconnect.
- Add login-time duplicate checks: reject connections when the same XUID is already connected (in addition to existing duplicate-name checks on Win64).
- Add inline compatibility comments around legacy/new identity coexistence paths for easier future maintenance.

* update: ensure uid.dat exists at startup in client mode for multiplayer

8 files changed, 388 insertions, 13 deletions

diff --git a/Minecraft.Client/ClientConnection.cpp b/Minecraft.Client/ClientConnection.cpp
index 2270433d..9b9bb86a 100644
--- a/Minecraft.Client/ClientConnection.cpp
+++ b/Minecraft.Client/ClientConnection.cpp
@@ -759,6 +759,18 @@ void ClientConnection::handleAddPlayer(shared_ptr<AddPlayerPacket> packet)
 			return;
 		}
 	}
+#ifdef _WINDOWS64
+	// Win64 keeps local-player identity separate from network smallId; also guard against creating
+	// a duplicate remote player for a local slot by checking the username directly.
+	for (unsigned int idx = 0; idx < XUSER_MAX_COUNT; ++idx)
+	{
+		if (minecraft->localplayers[idx] != NULL && minecraft->localplayers[idx]->name == packet->name)
+		{
+			app.DebugPrintf("AddPlayerPacket received for local player name %ls\n", packet->name.c_str());
+			return;
+		}
+	}
+#endif
 /*#ifdef _WINDOWS64
 	// On Windows64 all XUIDs are INVALID_XUID so the XUID check above never fires.
 	// packet->m_playerIndex is the server-assigned sequential index (set via LoginPacket),
@@ -800,8 +812,10 @@ void ClientConnection::handleAddPlayer(shared_ptr<AddPlayerPacket> packet)
 
 #ifdef _WINDOWS64
 	{
+		IQNetPlayer* matchedQNetPlayer = NULL;
 		PlayerUID pktXuid = player->getXuid();
 		const PlayerUID WIN64_XUID_BASE = (PlayerUID)0xe000d45248242f2e;
+		// Legacy compatibility path for peers still using embedded smallId XUIDs.
 		if (pktXuid >= WIN64_XUID_BASE && pktXuid < WIN64_XUID_BASE + MINECRAFT_NET_MAX_PLAYERS)
 		{
 			BYTE smallId = (BYTE)(pktXuid - WIN64_XUID_BASE);
@@ -809,13 +823,39 @@ void ClientConnection::handleAddPlayer(shared_ptr<AddPlayerPacket> packet)
 			if (np != NULL)
 			{
 				NetworkPlayerXbox* npx = (NetworkPlayerXbox*)np;
+				matchedQNetPlayer = npx->GetQNetPlayer();
+			}
+		}
+
+		// Current Win64 path: identify QNet player by name and attach packet XUID.
+		if (matchedQNetPlayer == NULL)
+		{
+			for (BYTE smallId = 0; smallId < MINECRAFT_NET_MAX_PLAYERS; ++smallId)
+			{
+				INetworkPlayer* np = g_NetworkManager.GetPlayerBySmallId(smallId);
+				if (np == NULL)
+					continue;
+
+				NetworkPlayerXbox* npx = (NetworkPlayerXbox*)np;
 				IQNetPlayer* qp = npx->GetQNetPlayer();
-				if (qp != NULL && qp->m_gamertag[0] == 0)
+				if (qp != NULL && _wcsicmp(qp->m_gamertag, packet->name.c_str()) == 0)
 				{
-					wcsncpy_s(qp->m_gamertag, 32, packet->name.c_str(), _TRUNCATE);
+					matchedQNetPlayer = qp;
+					break;
 				}
 			}
 		}
+
+		if (matchedQNetPlayer != NULL)
+		{
+			// Store packet-authoritative XUID on this network slot so later lookups by XUID
+			// (e.g. remove player, display mapping) work for both legacy and uid.dat clients.
+			matchedQNetPlayer->m_resolvedXuid = pktXuid;
+			if (matchedQNetPlayer->m_gamertag[0] == 0)
+			{
+				wcsncpy_s(matchedQNetPlayer->m_gamertag, 32, packet->name.c_str(), _TRUNCATE);
+			}
+		}
 	}
 #endif
 
@@ -985,6 +1025,8 @@ void ClientConnection::handleRemoveEntity(shared_ptr<RemoveEntitiesPacket> packe
 							qp->m_smallId = 0;
 							qp->m_isRemote = false;
 							qp->m_isHostPlayer = false;
+							// Clear resolved id to avoid stale XUID -> player matches after disconnect.
+							qp->m_resolvedXuid = INVALID_XUID;
 							qp->m_gamertag[0] = 0;
 							qp->SetCustomDataValue(0);
 						}
@@ -3956,4 +3998,4 @@ ClientConnection::DeferredEntityLinkPacket::DeferredEntityLinkPacket(shared_ptr<
 {
 	m_recievedTick = GetTickCount();
 	m_packet = packet;
-}
\ No newline at end of file
+}
diff --git a/Minecraft.Client/Common/Network/PlatformNetworkManagerStub.cpp b/Minecraft.Client/Common/Network/PlatformNetworkManagerStub.cpp
index 85531e47..80fbd98c 100644
--- a/Minecraft.Client/Common/Network/PlatformNetworkManagerStub.cpp
+++ b/Minecraft.Client/Common/Network/PlatformNetworkManagerStub.cpp
@@ -5,6 +5,7 @@
 #include "..\..\Xbox\Network\NetworkPlayerXbox.h"
 #ifdef _WINDOWS64
 #include "..\..\Windows64\Network\WinsockNetLayer.h"
+#include "..\..\Windows64\Windows64_Xuid.h"
 #include "..\..\Minecraft.h"
 #include "..\..\User.h"
 #include <iostream>
@@ -234,6 +235,7 @@ void CPlatformNetworkManagerStub::DoWork()
 				qnetPlayer->m_smallId = 0;
 				qnetPlayer->m_isRemote = false;
 				qnetPlayer->m_isHostPlayer = false;
+				qnetPlayer->m_resolvedXuid = INVALID_XUID;
 				qnetPlayer->m_gamertag[0] = 0;
 				qnetPlayer->SetCustomDataValue(0);
 				WinsockNetLayer::PushFreeSmallId(disconnectedSmallId);
@@ -354,7 +356,9 @@ void CPlatformNetworkManagerStub::HostGame(int localUsersMask, bool bOnlineGame,
 #ifdef _WINDOWS64
 	IQNet::m_player[0].m_smallId = 0;
 	IQNet::m_player[0].m_isRemote = false;
+	// world host is pinned to legacy host XUID to keep old player data compatibility.
 	IQNet::m_player[0].m_isHostPlayer = true;
+	IQNet::m_player[0].m_resolvedXuid = Win64Xuid::GetLegacyEmbeddedHostXuid();
 	IQNet::s_playerCount = 1;
 #endif
 
@@ -411,6 +415,8 @@ int CPlatformNetworkManagerStub::JoinGame(FriendSessionInfo* searchResult, int l
 	IQNet::m_player[0].m_smallId = 0;
 	IQNet::m_player[0].m_isRemote = true;
 	IQNet::m_player[0].m_isHostPlayer = true;
+	// Remote host still maps to legacy host XUID in mixed old/new sessions.
+	IQNet::m_player[0].m_resolvedXuid = Win64Xuid::GetLegacyEmbeddedHostXuid();
 	wcsncpy_s(IQNet::m_player[0].m_gamertag, 32, searchResult->data.hostName, _TRUNCATE);
 
 	WinsockNetLayer::StopDiscovery();
@@ -426,6 +432,8 @@ int CPlatformNetworkManagerStub::JoinGame(FriendSessionInfo* searchResult, int l
 	IQNet::m_player[localSmallId].m_smallId = localSmallId;
 	IQNet::m_player[localSmallId].m_isRemote = false;
 	IQNet::m_player[localSmallId].m_isHostPlayer = false;
+	// Local non-host identity is the persistent uid.dat XUID.
+	IQNet::m_player[localSmallId].m_resolvedXuid = Win64Xuid::ResolvePersistentXuid();
 
 	Minecraft* pMinecraft = Minecraft::GetInstance();
 	wcscpy_s(IQNet::m_player[localSmallId].m_gamertag, 32, pMinecraft->user->name.c_str());
diff --git a/Minecraft.Client/Extrax64Stubs.cpp b/Minecraft.Client/Extrax64Stubs.cpp
index 29865e40..9e693d1a 100644
--- a/Minecraft.Client/Extrax64Stubs.cpp
+++ b/Minecraft.Client/Extrax64Stubs.cpp
@@ -21,6 +21,7 @@
 #include "Windows64\Social\SocialManager.h"
 #include "Windows64\Sentient\DynamicConfigurations.h"
 #include "Windows64\Network\WinsockNetLayer.h"
+#include "Windows64\Windows64_Xuid.h"
 #elif defined __PSVITA__
 #include "PSVita\Sentient\SentientManager.h"
 #include "StatsCounter.h"
@@ -200,7 +201,15 @@ DWORD IQNetPlayer::GetCurrentRtt() { return 0; }
 bool IQNetPlayer::IsHost() { return m_isHostPlayer; }
 bool IQNetPlayer::IsGuest() { return false; }
 bool IQNetPlayer::IsLocal() { return !m_isRemote; }
-PlayerUID IQNetPlayer::GetXuid() { return (PlayerUID)(0xe000d45248242f2e + m_smallId); } // todo: restore to INVALID_XUID once saves support this
+PlayerUID IQNetPlayer::GetXuid()
+{
+	// Compatibility model:
+	// - Preferred path: use per-player resolved XUID populated from login/add-player flow.
+	// - Fallback path: keep legacy base+smallId behavior for peers/saves still on old scheme.
+	if (m_resolvedXuid != INVALID_XUID)
+		return m_resolvedXuid;
+	return (PlayerUID)(0xe000d45248242f2e + m_smallId);
+}
 LPCWSTR IQNetPlayer::GetGamertag() { return m_gamertag; }
 int IQNetPlayer::GetSessionIndex() { return m_smallId; }
 bool IQNetPlayer::IsTalking() { return false; }
@@ -226,6 +235,7 @@ void Win64_SetupRemoteQNetPlayer(IQNetPlayer * player, BYTE smallId, bool isHost
 	player->m_smallId = smallId;
 	player->m_isRemote = !isLocal;
 	player->m_isHostPlayer = isHost;
+	player->m_resolvedXuid = INVALID_XUID;
 	swprintf_s(player->m_gamertag, 32, L"Player%d", smallId);
 	if (smallId >= IQNet::s_playerCount)
 		IQNet::s_playerCount = smallId + 1;
@@ -285,8 +295,13 @@ IQNetPlayer* IQNet::GetPlayerByXuid(PlayerUID xuid)
 {
 	for (DWORD i = 0; i < s_playerCount; i++)
 	{
-		if (Win64_IsActivePlayer(&m_player[i], i) && m_player[i].GetXuid() == xuid) return &m_player[i];
+		if (!Win64_IsActivePlayer(&m_player[i], i))
+			continue;
+
+		if (m_player[i].GetXuid() == xuid)
+			return &m_player[i];
 	}
+	// Keep existing stub behavior: return host slot instead of NULL on miss.
 	return &m_player[0];
 }
 DWORD IQNet::GetPlayerCount()
@@ -301,7 +316,13 @@ DWORD IQNet::GetPlayerCount()
 QNET_STATE IQNet::GetState() { return _iQNetStubState; }
 bool IQNet::IsHost() { return s_isHosting; }
 HRESULT IQNet::JoinGameFromInviteInfo(DWORD dwUserIndex, DWORD dwUserMask, const INVITE_INFO * pInviteInfo) { return S_OK; }
-void IQNet::HostGame() { _iQNetStubState = QNET_STATE_SESSION_STARTING; s_isHosting = true; }
+void IQNet::HostGame()
+{
+	_iQNetStubState = QNET_STATE_SESSION_STARTING;
+	s_isHosting = true;
+	// Host slot keeps legacy XUID so old host player data remains addressable.
+	m_player[0].m_resolvedXuid = Win64Xuid::GetLegacyEmbeddedHostXuid();
+}
 void IQNet::ClientJoinGame()
 {
 	_iQNetStubState = QNET_STATE_SESSION_STARTING;
@@ -312,6 +333,7 @@ void IQNet::ClientJoinGame()
 		m_player[i].m_smallId = (BYTE)i;
 		m_player[i].m_isRemote = true;
 		m_player[i].m_isHostPlayer = false;
+		m_player[i].m_resolvedXuid = INVALID_XUID;
 		m_player[i].m_gamertag[0] = 0;
 		m_player[i].SetCustomDataValue(0);
 	}
@@ -326,6 +348,7 @@ void IQNet::EndGame()
 		m_player[i].m_smallId = (BYTE)i;
 		m_player[i].m_isRemote = false;
 		m_player[i].m_isHostPlayer = false;
+		m_player[i].m_resolvedXuid = INVALID_XUID;
 		m_player[i].m_gamertag[0] = 0;
 		m_player[i].SetCustomDataValue(0);
 	}
@@ -575,10 +598,13 @@ void				C_4JProfile::GetXUID(int iPad, PlayerUID * pXuid, bool bOnlineXuid)
 		*pXuid = INVALID_XUID;
 		return;
 	}
+	// LoginPacket reads this value as client identity:
+	// - host keeps legacy host XUID for world compatibility
+	// - non-host uses persistent uid.dat-backed XUID
 	if (IQNet::s_isHosting)
-		*pXuid = 0xe000d45248242f2e;
+		*pXuid = Win64Xuid::GetLegacyEmbeddedHostXuid();
 	else
-		*pXuid = 0xe000d45248242f2e + WinsockNetLayer::GetLocalSmallId();
+		*pXuid = Win64Xuid::ResolvePersistentXuid();
 #else
 	* pXuid = 0xe000d45248242f2e + iPad;
 #endif
diff --git a/Minecraft.Client/Minecraft.cpp b/Minecraft.Client/Minecraft.cpp
index b87745f0..7ae206cf 100644
--- a/Minecraft.Client/Minecraft.cpp
+++ b/Minecraft.Client/Minecraft.cpp
@@ -52,6 +52,7 @@
 #include "..\Minecraft.World\net.minecraft.world.level.dimension.h"
 #include "..\Minecraft.World\net.minecraft.world.item.h"
 #include "..\Minecraft.World\Minecraft.World.h"
+#include "Windows64\Windows64_Xuid.h"
 #include "ClientConnection.h"
 #include "..\Minecraft.World\HellRandomLevelSource.h"
 #include "..\Minecraft.World\net.minecraft.world.entity.animal.h"
@@ -1038,6 +1039,19 @@ shared_ptr<MultiplayerLocalPlayer> Minecraft::createExtraLocalPlayer(int idx, co
 		PlayerUID playerXUIDOnline = INVALID_XUID;
 		ProfileManager.GetXUID(idx,&playerXUIDOffline,false);
 		ProfileManager.GetXUID(idx,&playerXUIDOnline,true);
+#ifdef _WINDOWS64
+		// Compatibility rule for Win64 id migration
+		// host keeps legacy host XUID, non-host uses persistent uid.dat XUID.
+		INetworkPlayer *localNetworkPlayer = g_NetworkManager.GetLocalPlayerByUserIndex(idx);
+		if(localNetworkPlayer != NULL && localNetworkPlayer->IsHost())
+		{
+			playerXUIDOffline = Win64Xuid::GetLegacyEmbeddedHostXuid();
+		}
+		else
+		{
+			playerXUIDOffline = Win64Xuid::ResolvePersistentXuid();
+		}
+#endif
 		localplayers[idx]->setXuid(playerXUIDOffline);
 		localplayers[idx]->setOnlineXuid(playerXUIDOnline);
 		localplayers[idx]->setIsGuest(ProfileManager.IsGuest(idx));
@@ -4299,6 +4313,19 @@ void Minecraft::setLevel(MultiPlayerLevel *level, int message /*=-1*/, shared_pt
 				playerXUIDOnline.setForAdhoc();
 			}
 #endif
+#ifdef _WINDOWS64
+			// On Windows, the implementation has been changed to use a per-client pseudo XUID based on `uid.dat`.  
+			// To maintain player data compatibility with existing worlds, the world host (the first player) will use the previous embedded pseudo XUID.
+			INetworkPlayer *localNetworkPlayer = g_NetworkManager.GetLocalPlayerByUserIndex(iPrimaryPlayer);
+			if(localNetworkPlayer != NULL && localNetworkPlayer->IsHost())
+			{
+				playerXUIDOffline = Win64Xuid::GetLegacyEmbeddedHostXuid();
+			}
+			else
+			{
+				playerXUIDOffline = Win64Xuid::ResolvePersistentXuid();
+			}
+#endif
 			player->setXuid(playerXUIDOffline);
 			player->setOnlineXuid(playerXUIDOnline);
 
@@ -4481,6 +4508,18 @@ void Minecraft::respawnPlayer(int iPad, int dimension, int newEntityId)
 	PlayerUID playerXUIDOnline = INVALID_XUID;
 	ProfileManager.GetXUID(iTempPad,&playerXUIDOffline,false);
 	ProfileManager.GetXUID(iTempPad,&playerXUIDOnline,true);
+#ifdef _WINDOWS64
+	// Same compatibility rule as create/init paths.
+	INetworkPlayer *localNetworkPlayer = g_NetworkManager.GetLocalPlayerByUserIndex(iTempPad);
+	if(localNetworkPlayer != NULL && localNetworkPlayer->IsHost())
+	{
+		playerXUIDOffline = Win64Xuid::GetLegacyEmbeddedHostXuid();
+	}
+	else
+	{
+		playerXUIDOffline = Win64Xuid::ResolvePersistentXuid();
+	}
+#endif
 	player->setXuid(playerXUIDOffline);
 	player->setOnlineXuid(playerXUIDOnline);
 	player->setIsGuest( ProfileManager.IsGuest(iTempPad) );
diff --git a/Minecraft.Client/PendingConnection.cpp b/Minecraft.Client/PendingConnection.cpp
index b50669e5..72b8f382 100644
--- a/Minecraft.Client/PendingConnection.cpp
+++ b/Minecraft.Client/PendingConnection.cpp
@@ -161,6 +161,23 @@ void PendingConnection::handleLogin(shared_ptr<LoginPacket> packet)
 	//if (true)// 4J removed !server->onlineMode)
 	bool sentDisconnect = false;
 
+	// Use the same Xuid choice as handleAcceptedLogin (offline first, online fallback).
+	// 
+	PlayerUID loginXuid = packet->m_offlineXuid;
+	if (loginXuid == INVALID_XUID) loginXuid = packet->m_onlineXuid;
+
+	bool duplicateXuid = false;
+	if (loginXuid != INVALID_XUID && server->getPlayers()->getPlayer(loginXuid) != nullptr)
+	{
+		duplicateXuid = true;
+	}
+	else if (packet->m_onlineXuid != INVALID_XUID &&
+		packet->m_onlineXuid != loginXuid &&
+		server->getPlayers()->getPlayer(packet->m_onlineXuid) != nullptr)
+	{
+		duplicateXuid = true;
+	}
+
 	if( sentDisconnect )
 	{
 		// Do nothing
@@ -169,6 +186,12 @@ void PendingConnection::handleLogin(shared_ptr<LoginPacket> packet)
 	{
 		disconnect(DisconnectPacket::eDisconnect_Banned);
 	}
+	else if (duplicateXuid)
+	{
+		// if same XUID already in use by another player so disconnect this one.
+		app.DebugPrintf("Rejecting duplicate xuid for name: %ls\n", name.c_str());
+		disconnect(DisconnectPacket::eDisconnect_Banned);
+	}
 #ifdef _WINDOWS64
 	else if (g_bRejectDuplicateNames)
 	{
diff --git a/Minecraft.Client/PlayerList.cpp b/Minecraft.Client/PlayerList.cpp
index c1ccfe14..42724a4f 100644
--- a/Minecraft.Client/PlayerList.cpp
+++ b/Minecraft.Client/PlayerList.cpp
@@ -18,6 +18,7 @@
 #include "..\Minecraft.World\ArrayWithLength.h"
 #include "..\Minecraft.World\net.minecraft.network.packet.h"
 #include "..\Minecraft.World\net.minecraft.network.h"
+#include "Windows64\Windows64_Xuid.h"
 #include "..\Minecraft.World\Pos.h"
 #include "..\Minecraft.World\ProgressListener.h"
 #include "..\Minecraft.World\HellRandomLevelSource.h"
@@ -106,11 +107,18 @@ void PlayerList::placeNewPlayer(Connection *connection, shared_ptr<ServerPlayer>
 	}
 #endif
 #ifdef _WINDOWS64
-	if (networkPlayer != NULL && !networkPlayer->IsLocal())
+	if (networkPlayer != NULL)
 	{
 		NetworkPlayerXbox* nxp = (NetworkPlayerXbox*)networkPlayer;
 		IQNetPlayer* qnp = nxp->GetQNetPlayer();
-		wcsncpy_s(qnp->m_gamertag, 32, player->name.c_str(), _TRUNCATE);
+		if (qnp != NULL)
+		{
+			if (!networkPlayer->IsLocal())
+			{
+				wcsncpy_s(qnp->m_gamertag, 32, player->name.c_str(), _TRUNCATE);
+			}
+			qnp->m_resolvedXuid = player->getXuid();
+		}
 	}
 #endif
 	// 4J Stu - TU-1 hotfix
@@ -520,12 +528,20 @@ shared_ptr<ServerPlayer> PlayerList::getPlayerForLogin(PendingConnection *pendin
 	player->setOnlineXuid( onlineXuid ); // 4J Added
 #ifdef _WINDOWS64
 	{
+		// Use packet-supplied identity from LoginPacket.
+		// Do not recompute from name here: mixed-version clients must stay compatible.
 		INetworkPlayer* np = pendingConnection->connection->getSocket()->getPlayer();
 		if (np != NULL)
 		{
-			PlayerUID realXuid = np->GetUID();
-			player->setXuid(realXuid);
-			player->setOnlineXuid(realXuid);
+			player->setOnlineXuid(np->GetUID());
+
+			// Backward compatibility: when Minecraft.Client is hosting, keep the first
+			// host player on the legacy embedded host XUID (base + 0).
+			// This preserves pre-migration host playerdata in existing worlds.
+			if (np->IsHost())
+			{
+				player->setXuid(Win64Xuid::GetLegacyEmbeddedHostXuid());
+			}
 		}
 	}
 #endif
diff --git a/Minecraft.Client/Windows64/Windows64_Minecraft.cpp b/Minecraft.Client/Windows64/Windows64_Minecraft.cpp
index 1146b86d..fa4b8366 100644
--- a/Minecraft.Client/Windows64/Windows64_Minecraft.cpp
+++ b/Minecraft.Client/Windows64/Windows64_Minecraft.cpp
@@ -42,6 +42,7 @@
 #include "..\..\Minecraft.World\OldChunkStorage.h"
 #include "Common/PostProcesser.h"
 #include "Network\WinsockNetLayer.h"
+#include "Windows64_Xuid.h"
 
 #include "Xbox/resource.h"
 
@@ -1221,6 +1222,12 @@ int APIENTRY _tWinMain(_In_ HINSTANCE hInstance,
 	Win64LaunchOptions launchOptions = ParseLaunchOptions();
 	ApplyScreenMode(launchOptions.screenMode);
 
+	// Ensure uid.dat exists from startup in client mode (before any multiplayer/login path).
+	if (!launchOptions.serverMode)
+	{
+		Win64Xuid::ResolvePersistentXuid();
+	}
+
 	// If no username, let's fall back
 	if (g_Win64Username[0] == 0)
 	{
diff --git a/Minecraft.Client/Windows64/Windows64_Xuid.h b/Minecraft.Client/Windows64/Windows64_Xuid.h
new file mode 100644
index 00000000..93577751
--- /dev/null
+++ b/Minecraft.Client/Windows64/Windows64_Xuid.h
@@ -0,0 +1,214 @@
+#pragma once
+
+#ifdef _WINDOWS64
+
+#include <string>
+#include <cstdio>
+#include <cstdlib>
+#include <cerrno>
+#include <cstring>
+#include <Windows.h>
+
+namespace Win64Xuid
+{
+	inline PlayerUID GetLegacyEmbeddedBaseXuid()
+	{
+		return (PlayerUID)0xe000d45248242f2eULL;
+	}
+
+	inline PlayerUID GetLegacyEmbeddedHostXuid()
+	{
+		// Legacy behavior used "embedded base + smallId"; host was always smallId 0.
+		// We intentionally keep this value for host/self compatibility with pre-migration worlds.
+		return GetLegacyEmbeddedBaseXuid();
+	}
+
+	inline bool IsLegacyEmbeddedRange(PlayerUID xuid)
+	{
+		// Old Win64 XUIDs were not persistent and always lived in this narrow base+smallId range.
+		// Treat them as legacy/non-persistent so uid.dat values never collide with old slot IDs.
+		const PlayerUID base = GetLegacyEmbeddedBaseXuid();
+		return xuid >= base && xuid < (base + MINECRAFT_NET_MAX_PLAYERS);
+	}
+
+	inline bool IsPersistedUidValid(PlayerUID xuid)
+	{
+		return xuid != INVALID_XUID && !IsLegacyEmbeddedRange(xuid);
+	}
+
+
+	// ./uid.dat
+	inline bool BuildUidFilePath(char* outPath, size_t outPathSize)
+	{
+		if (outPath == NULL || outPathSize == 0)
+			return false;
+
+		outPath[0] = 0;
+
+		char exePath[MAX_PATH] = {};
+		DWORD len = GetModuleFileNameA(NULL, exePath, MAX_PATH);
+		if (len == 0 || len >= MAX_PATH)
+			return false;
+
+		char* lastSlash = strrchr(exePath, '\\');
+		if (lastSlash != NULL)
+		{
+			*(lastSlash + 1) = 0;
+		}
+
+		if (strcpy_s(outPath, outPathSize, exePath) != 0)
+			return false;
+		if (strcat_s(outPath, outPathSize, "uid.dat") != 0)
+			return false;
+
+		return true;
+	}
+
+	inline bool ReadUid(PlayerUID* outXuid)
+	{
+		if (outXuid == NULL)
+			return false;
+
+		char path[MAX_PATH] = {};
+		if (!BuildUidFilePath(path, MAX_PATH))
+			return false;
+
+		FILE* f = NULL;
+		if (fopen_s(&f, path, "rb") != 0 || f == NULL)
+			return false;
+
+		char buffer[128] = {};
+		size_t readBytes = fread(buffer, 1, sizeof(buffer) - 1, f);
+		fclose(f);
+
+		if (readBytes == 0)
+			return false;
+
+		// Compatibility: earlier experiments may have written raw 8-byte uid.dat.
+		if (readBytes == sizeof(unsigned __int64))
+		{
+			unsigned __int64 raw = 0;
+			memcpy(&raw, buffer, sizeof(raw));
+			PlayerUID parsed = (PlayerUID)raw;
+			if (IsPersistedUidValid(parsed))
+			{
+				*outXuid = parsed;
+				return true;
+			}
+		}
+
+		buffer[readBytes] = 0;
+		char* begin = buffer;
+		while (*begin == ' ' || *begin == '\t' || *begin == '\r' || *begin == '\n')
+		{
+			++begin;
+		}
+
+		errno = 0;
+		char* end = NULL;
+		unsigned __int64 raw = _strtoui64(begin, &end, 0);
+		if (begin == end || errno != 0)
+			return false;
+
+		while (*end == ' ' || *end == '\t' || *end == '\r' || *end == '\n')
+		{
+			++end;
+		}
+		if (*end != 0)
+			return false;
+
+		PlayerUID parsed = (PlayerUID)raw;
+		if (!IsPersistedUidValid(parsed))
+			return false;
+
+		*outXuid = parsed;
+		return true;
+	}
+
+	inline bool WriteUid(PlayerUID xuid)
+	{
+		char path[MAX_PATH] = {};
+		if (!BuildUidFilePath(path, MAX_PATH))
+			return false;
+
+		FILE* f = NULL;
+		if (fopen_s(&f, path, "wb") != 0 || f == NULL)
+			return false;
+
+		int written = fprintf_s(f, "0x%016llX\n", (unsigned long long)xuid);
+		fclose(f);
+		return written > 0;
+	}
+
+	inline unsigned __int64 Mix64(unsigned __int64 x)
+	{
+		x += 0x9E3779B97F4A7C15ULL;
+		x = (x ^ (x >> 30)) * 0xBF58476D1CE4E5B9ULL;
+		x = (x ^ (x >> 27)) * 0x94D049BB133111EBULL;
+		return x ^ (x >> 31);
+	}
+
+	inline PlayerUID GeneratePersistentUid()
+	{
+		// Avoid rand_s dependency: mix several Win64 runtime values into a 64-bit seed.
+		FILETIME ft = {};
+		GetSystemTimeAsFileTime(&ft);
+		unsigned __int64 t = (((unsigned __int64)ft.dwHighDateTime) << 32) | ft.dwLowDateTime;
+
+		LARGE_INTEGER qpc = {};
+		QueryPerformanceCounter(&qpc);
+
+		unsigned __int64 seed = t;
+		seed ^= (unsigned __int64)qpc.QuadPart;
+		seed ^= ((unsigned __int64)GetCurrentProcessId() << 32);
+		seed ^= (unsigned __int64)GetCurrentThreadId();
+		seed ^= (unsigned __int64)GetTickCount();
+		seed ^= (unsigned __int64)(size_t)&qpc;
+		seed ^= (unsigned __int64)(size_t)GetModuleHandleA(NULL);
+
+		unsigned __int64 raw = Mix64(seed) ^ Mix64(seed + 0xA0761D6478BD642FULL);
+		raw ^= 0x8F4B2D6C1A93E705ULL;
+		raw |= 0x8000000000000000ULL;
+
+		PlayerUID xuid = (PlayerUID)raw;
+		if (!IsPersistedUidValid(xuid))
+		{
+			raw ^= 0x0100000000000001ULL;
+			xuid = (PlayerUID)raw;
+		}
+
+		if (!IsPersistedUidValid(xuid))
+		{
+			// Last-resort deterministic fallback for pathological cases.
+			xuid = (PlayerUID)0xD15EA5E000000001ULL;
+		}
+
+		return xuid;
+	}
+
+	inline PlayerUID ResolvePersistentXuid()
+	{
+		// Process-local cache: uid.dat is immutable during runtime and this path is hot.
+		static bool s_cached = false;
+		static PlayerUID s_xuid = INVALID_XUID;
+
+		if (s_cached)
+			return s_xuid;
+
+		PlayerUID fileXuid = INVALID_XUID;
+		if (ReadUid(&fileXuid))
+		{
+			s_xuid = fileXuid;
+			s_cached = true;
+			return s_xuid;
+		}
+
+		// First launch on this client: generate once and persist to uid.dat.
+		s_xuid = GeneratePersistentUid();
+		WriteUid(s_xuid);
+		s_cached = true;
+		return s_xuid;
+	}
+}
+
+#endif